At least one species of teleost fish, the marine catfish (Arius felis), uses a form of echolocation to detect the presence of nearby obstacles. Low frequency (100 Hz) sound pulses are used for obstacle detection as well as for social communication. By means of behavioral techniques, the hearing capacities of this species are being determined, particularly those parameters required for low frequency echolocation. Thus far, auditory sensitivity, signal/noise ratios, and frequency discrimination limens have been found to be particularly good in the low frequency range, 100 to 200 Hz. Studies to determine the range and sensitivity of the echolocation system are in progress. Behavioral and other experimental techniques will be used to evaluate the relative roles of the swim bladder-inner ear hearing pathway with comparisons with other possible pathways, i.e., direct inner ear sound detection and the involvement of the lateral line system. The fine structure and other morphology of the inner ear in this species has several unusual features, and the relation of these features to hearing capacities is being explored. Echolocation in the sea catfish utilizes low frequency sounds and the emitted sounds become mixed with reflected and ambient sounds. This echolocating system is considered primitive, as compared to that of bats and dolphins. This system, however, is actually closer in its operation and requirements to natural echolocating abilities that exist in human beings. The study of primitive forms of echolocation should provide significant information on the human capacities for such acoustic orientation.